Inflatable packers have numerous uses in downhole applications. They are used externally on liners to seal against a borehole wall. They are also used to isolate different zones in a wellbore for production. Inflatables can be designed for passage through tubing or can be carried on tubing or externally on a liner. In some embodiments, the inflatables are fairly lengthy, and the manner in which they inflate can be important. To this end, long inflatables have been designed which inflate progressively to ensure that the entire length of the element is seated against the casing or borehole wall. Typical of such designs are U.S. Pat. Nos. 4,781,249; 4,897,139; and 4,967,846. In these patents, the element is made in one of several unique manners to accomplish progressive inflation. One way is to change its thickness along its length or the properties of the rubber element.
Typically, inflatables of the past have been set by applied fluid pressure in the tubing or liner, or by use of straddle tools. Generally speaking, these packers would have an opening in their mandrel to allow the pressurized fluid, i.e., drilling mud or a cementitious material, to enter under pressure between the mandrel and the element for the purposes of inflation. A straddle tool seeks to straddle the opening in the mandrel so that the inflating fluid can be spotted directly into the annular space between the mandrel and the inflating element. Some disadvantages of using straddle tools relates to spillage at the conclusion of the inflating step. It is disadvantageous to have the excess inflating material remain in the wellbore, particularly if it hardens over time. Thus, circulation or reverse circulation may be necessary to remove such material from the wellbore. Another alternative is to use a material for inflation which is pushed into the annular space between the mandrel and the inflatable element by virtue of wiper plugs which are pumped downhole. Eventually, the wiper plugs are drilled out after the inflation process concludes.
These configurations for inflatable packers had several distinct disadvantages. First of all, the opening in the mandrel wall necessary to allow admission of inflation fluid presented a potential leakpath through the tubing string that supports the inflatable. Additionally, introduction of fill fluids through the tubing string created problems of cleaning out residual material. Alternatively, the drilling out of wiper plugs was also time-consuming.
The prior techniques to secure the features of progressive inflation dealt with modification of the inflatable element. These techniques were expensive and, in some cases, increased the profile of the packer, making it more difficult to use it in certain applications.
Prior designs involving openings in the mandrel also required a valving arrangement to exclude fluid from under the element until a predetermined differential pressure on the element is reached. Another technique of inflating prior packers is to run a control line down to the packer and inflate the element using the control line.
A technique of making casing patches has been developed which involves taking open-ended corrugated pipe, placing it in the wellbore, and mechanically expanding it into contact with the casing. In this technique, a segment of casing is axially corrugated. The application of a force to the corrugated casing forces it outwardly to assume the rounded shape, using forces that are below the burst pressure of the rounded tube. These applications have generally been on fairly short segments of casing, generally in the order of 10-20 ft., and have seen application exclusively as casing patches. Cross-sections, as shown in FIG. 5, have been used in making a casing patch. Homco offers a device which can expand such corrugated tubes against casing to make a short patch.
The object of the present invention employs a mandrel which is made from such corrugated tubing. In the preferred embodiment, a material is placed between the mandrel and the element such that when forces are applied to the mandrel, the intermediate fluid pushes out with the walls of the mandrel against the inflatable element. This technique eliminates openings in the mandrel wall. It further meets the objective of reducing the profile of the inflatable to facilitate its placement through small openings. The objective of progressive inflation is also accomplished by manipulation of the configuration of the mandrel. By a careful selection of the intermediary material between the mandrel and the element, a permanently set packer can be achieved. Another objective of the invention is to allow any excess applied pressure force of the inflation fluid against the element to be relieved from the annular space under the element during the expansion process of the mandrel. These and other objectives of the present invention will be made more clear from a review of the description of the preferred embodiment.